Galling is a form of adhesive wear which is most damaging at relatively high stresses. Seizure of mating components is frequently a result of severe galling and usually takes place early in the life of equipment. Threaded components are particularly noted for their seizure tendencies.
When two surfaces are loaded together, the coacting surfaces may form strong bonds due to the locally high pressure and heat generated by subsequent motion. If these bonds sever at the interface, little damage occurs and the parts run smoothly; on the other hand, if fracture occurs in either material, gross damage results. This damage is termed galling.
In the past, organic and inorganic lubricants have been employed to resist galling failure. Common methods previously employed have included metallic electroplatings of tin and/or zinc, which may cause serious environmental waste disposal problems. Phosphate conversion coatings have also been used which also have environmental disposal problems and are limited to the type of material substrate to which it can be applied. Base lubricant films, such as API modified thread compound commonly used on threaded connectors allow slippage between moving parts as long as the moving parts are not chemically, thermally, or physically destroyed or displaced. However, after the lubricant is penetrated, cold welding of the active surfaces in contact with each other begins. Cold weldings result in the destruction by galling of the mating surfaces.
A need has thus arisen for a method for resisting galling of coacting metal-to-metal surfaces, whether they be sealing surfaces, shoulders or thread elements.